Getting the female perspective across in mainstream cinema has been a long slog. It is still unusual to find a young woman behind the camera as well as in front of it. But, at 27, director MJ Delaney has now put the unvarnished truth on screen. Her new film, Powder Room, which stars Sheridan Smith and Jaime Winstone in an adaptation of a play by Rachel Hirons, expresses an unapologetic disregard for the meek, self-effacing heroines that commonly earn screen time.

Yet this was not what drew her to the story. “I just thought it was funny, honest and truthful, as well as refreshingly realistic and grimy,” she told the Observer this weekend.

The idea that the plot, which revolves around the ladies’ loo in a nightclub, might alienate male cinema-goers did not occur to her. “I never considered whether I would be alienating men. If you were to make a film with a predominantly male cast that was set in a largely male environment, nobody would ask you if you were worried you were going to alienate the female audience.”

Delaney came to fame in an uncompromisingly modern way. Although she won a double first in English at Oxford, she got into film-making by making a spoof music video that went viral with 2.5 million YouTube viewers three years ago. Her Newport State of Mind was an engaging parody of the celebrated Alicia Keys and Jay-Z track, Empire State of Mind. MJ’s rap lyrics about the Welsh town were more parochial: “We don’t even have a Pizza Hut. Famous for cheese and chips.”

Her impulse to make Hirons’s play, When Women Wee, into a film was as much driven by her desire to see a different kind of world on screen as to give young women a new voice. “Girls all know about the toilet as a space in which you take refuge,” she said. “But I was keen on clubbing when I was growing up and I wanted to show some of that.”

Finding the right soundtrack became as important as discovering new angles from which to film a nightclub loo. “We looked for music that was new. We trawled the internet for tracks we liked and then approached the people behind them. On the whole, when we said what we were doing they were keen to be involved.” There is at least 80 minutes of new music in Powder Room, she thinks, and there is a resident club band too: Fake Club, featuring Rosie Bones, who is played by her old friend Rosie Oddie, daughter of Bill, the bird expert and former Goodie.

“I’m a Camden girl, so I like rockier music than most club sounds, but you cannot escape electronic dance music, and I love a rave as well, whether it’s a ridiculous rave in Vauxhall that goes on until 6am or a warehouse party in east London.”

The film’s frank sexual language and the attitudes it portrays towards drugs have already attracted attention. Delaney says she wanted to show a different side of a British city, one without a romantic sheen. “When you see London on film, it tends to be very Woody Allen-ish, or else the way Richard Curtis does it: both very white and very middleclass. But here we actually tend to have a more interesting scruffy and grimy aesthetic which I think can also be very beautiful. We don’t often see it on screen, though.”

Young Britons are not as smart as New Yorkers or Parisians when they go out, she argues. And happy to dress down a little for the part are some leading actors: appearing alongside Smith and Winstone are Kate Nash and Oona Chaplin, who play the two sophisticates that Smith’s character, Sam, is vainly trying to impress. “We never thought in a million years that we’d get someone of the calibre of Sheridan Smith,” said Delaney, who was operating on a small budget. “When we were casting it, she was on in the West End in Ibsen’s Hedda Gabler, shooting herself in the head every night in this very serious play. When this new script landed on her dressing room table I think we were lucky that it was the polar opposite of what she was doing – just a bunch of girls out for the night.”

Winstone took the part because she and Smith had always wanted to work together. “Jaime plays Sam’s old friend Chanel, who is like the moral compass of the story, although she doesn’t necessarily seem it at the beginning.”

Nightclubs, Delaney concedes, can be tough and even dangerous places for young women. But this is not what her film is about. If it is a feminist piece it is because it allows women to be themselves. “There is a lot of vulnerability on show in the film. This is not a fun night out for anyone. In fact it’s an awful night out, although it is funny. I don’t think this story is about the issues that might come up for women in a nightclub, things like rape or violence. Because the male characters are so peripheral and unimportant in this story, it’s not about any of that.”

The original inspiration for the play was the overheard remarks of two women in frank discussion about anal sex. “And that conversation is pretty much verbatim in the film. The powder room is a sanctuary and a place where you can see a particular side of female friendship.”

Unlike the comic hen night caricatures of competitive women often featured in scenes set in the ladies, in Powder Room Delaney wanted to bring out a sense of the loyalty that women develop. “Maybe it comes from the maternal instinct; something about looking after each other. There is a love that exists in a female friendship. I’m not saying it’s better from male friendship, but it is different, I think.”